Ok lets translate this words into maths. Let the cost of the shirt be x.
[tex]75\%x = \frac{1}{2} x + 4[/tex]
Now lets solve for x but first turn the percent and fraction to decimal
[tex]0.75x = 0.5x + 4 \\ 0.75x - 0.5x = 4 \\ .25x = 4 \\ .25x \times 4 = 4 \times 4 \\ x = 16[/tex]
The cost of the shirt is $16.00